How to static_assert if a constexpr number is in a constexpr array at compile time?-CodePudding

I want to check if a constexpr number is in a constexpr array at compile time. If not, stop compiling. My ideographic example:

constexpr void PinValid(uint8_t Pin)
{
    constexpr uint8_t ValidPins[] = {2, 3, 18, 19, 20, 21};
    for (uint8_t P : ValidPins)
        if (Pin == P)
            return;
    static_assert(false);
}

This code fails for any given Pin even if it is in the ValidPins.

CodePudding user response：

You can write like this:

constexpr bool PinValid(uint8_t Pin)
{
    constexpr uint8_t ValidPins[] = {2, 3, 18, 19, 20, 21};
    for (uint8_t P : ValidPins)
        if (Pin == P)
            return true;
    return false;
}

static_assert(PinValid(3));

CodePudding user response：

I'd like to add the following solution, even if the quality of the error message isn't as good as in the example in the other answer, on some compilers.

In c 20 you can throw an exception in a constexpr function which will result in a compilation error if it gets thrown at compile-time:

#include <cstdint>
#include <array>
#include <stdexcept>


consteval void PinValid(uint8_t Pin)
{
    constexpr uint8_t ValidPins[] = {2, 3, 18, 19, 20, 21};
    for (uint8_t P : ValidPins)
        if (Pin == P)
            return;
    throw std::invalid_argument("not a valid pin");
}

int main() {
    PinValid(20);
    PinValid(5); // Does not compile
}

See the example on Compiler Explorer. Notice that it currently doesn't work on clang, though